At the base of the 1st Battalion 5th US Marine Expeditionary Brigade at Garamsir in south Helmand they have a slogan on their T-shirts guaranteed to enrage Caroline Lucas and Simon Jenkins, two of Cif's most recent commentators on Afghanistan.

"Just do Marja" it reads. Marja is a quilt of small fertile plots just south and west of Lashkar Gah, the current provincial capital of Helmand. Like the irrigation channels that feed the fields of Marja, Lashkar Gah is largely the creation of a huge project by the US Agency for International Development (USAID) that made Helmand the bread basket of Afghanistan, and a magnet for tourism even.

Marja has become one of the big poppy growing plots of the world. Today it is largely under Taliban control, who run their "parallel government" there by night – which means robbing the farmers in the name of Islamic taxation, closing schools and demanding tribute in food, warm clothing, and young recruits for their jihad. It is also a centre for assembling IED roadside bombs, which they lay with astonishing deftness and speed.

Marja will be the first target of the Marine Expeditionary Unit now expected here before Christmas as a result of President Obama's anticipated announcement that he will send an extra 30,000 US military personnel to Afghanistan for the next two years. Squeezing Taliban out of Marja, and then Nad-e-Ali to the north, will remove the threat to commerce and farming along the west bank of the Helmand river.

Lashkar Gah is thriving and buzzing, compared with two years ago, when I was last here. The bazaars are booming full of all kinds of produce, a new line in iron bedsteads, small wheat-milling machines, and hundreds of motorbikes – most made in kits in China and assembled in Iran. Farmers and merchants now travel to Gereshk to the north and to Kandahar, less than three hours away. They say the roads are pretty safe, bar the risk of the odd rogue roadblock manned by Taliban or renegade Afghan police.

Lashkar Gah is at the centre of a security bubble or "protected development area" – a key concept of the "ink spot" approach of counter-insurgency theory and practice, recently retooled by General Stanley McChrystal. You take the main centres, such as Lashkar Gah, Garmsir, Gereshk and Musa Qala in Helmand, and protect them with international and then local forces. Confidence and commerce grow, and in time the different areas link together.

The problem, however, is that Afghanistan today defies all such generalities: the whole story is a patchwork quilt, a mosaic, of quirky and contradictory detail. Security and commerce, and even schooling and health, are visibly improving in many parts of Helmand, till now dubbed Afghanistan's most violent province. The Americans and the British are not being "defeated", though they are facing casualties. But to declare any kind of victory would be daft and dangerous. While there are signs of improvement in Helmand, elsewhere there is more than enough evidence that things are getting worse – as Carlotta Gall's report from Kunduz in today's New York Times highlights.

The McChrystal counter-insurgency is already being implemented, and showing signs of working particularly here in central Helmand. Roads are being secured, clinics and schools opened, courts and local councils set up. Communities are swinging from Taliban loyalty to supporting the government, but after nearly 50 years of war and violence they're hedging their bets. Almost all generalisations from the pundits and panjandrums in London, Washington an all points north seem vapid before the complexity of the facts here on the ground.

This struck most forcibly when yesterday I visited Nawa, between Lashkar Gah and Garmsir. It's not so much a one-horse town as a one-ditch town, with its bazaar strung out on a dirt and tarmac track alongside a slow-running, but remarkably clear irrigation ditch.

Until June the place was home to 60 British soldiers training a company of Afghan army troops. They were holed up in the barracks where they exchanged fire with Taliban in the surrounding orchards and bazaar on a daily basis. Last July the US marines arrived, staked out the place with a company of 300 troops, and a fortnight later drove the Taliban off with a full battalion attack of more than 1,000 ground forces with air and helicopter support.

Today the bazaar is booming. On the eve of Eid, the festival of joy and celebration at the end of the hajj, more than 80 shops were open – the Taliban had closed all but about six – selling fresh fruit, sweets, mobile phones, and the electricity from a sun panel to power them. The township has its own community council. But seven weeks ago the Taliban kidnapped its head, then executed him out in the desert, and shot two other councillors in their office. At first the rest of the council stayed away, but lately most meetings get a quorum of 25 out of the remaining 42.

"Every day of peace is like Eid," Haj Mohammed Khan, the clerk to the council told me. He continued:

The marines brought peace because the British didn't have the numbers. If you go away again, the violence will be much worse. There will be a disaster, the world will come here again to fight in a really big war.
You left twice before – and let in the mujahideen and what came after. This time it will be far worse.

His words had a strange echo from Captain Brian Huysman of Charlie Company, 1st Battalion 5th Marines, whose 70 men are helping the rebuilding and renovation of the council offices and barracks at Nawa. He was asked by a colleague to compare his experience of Helmand with two tours in Iraq at Fallujah and then running a community centre for five months of 2005 in Ramadi ("a complete failure" in his words). He said:

At least I get the feeling we're winning, which I didn't there. The answer is in the approach to the people, getting in among the people, and here we eat in the bazaar every day. Get the approach right and then the force numbers right, that's the key.

"Yes, and that's the way we will be doing things for the next 15 or 20 years, and it's what every grunt and general needs to learn now," added his colleague Major Val Jackson, a US marines civil affairs officer.

Nawa, last year the heart of Helmand darkness, now seems to point the way to the future for the Afghans here, and to how the international support agencies, not just the military, can help and then get out.

The problems are still complex and enormous, not least the issues raised by the complexion of the Karzai administration, its legitimacy and the corrosive nature of the drugs trade. But there are signs of forward momentum, and this should be helped by the modest reinforcements of troops and aid due to be announced next week. The task has been likened to by an NGO colleague to her experiences in working in Cambodia after the psychopathic rule of the Khmer Rouge. "So much was completely broken here by the mujahideen civil war and then the Taliban."

To quit now, as Jenkins and Lucas recommend, would be sheer folly – and a folly which would have direct impact on homeland Europe, UK and America even. I agree with Jenkins on one thing: Whitehall, Westminster and large parts of Washington are blanketed in a cloud of passivity and pessimism about Afghanistan. The complexities of the picture on the ground elude commentators who come her in flying visits with high powered delegations of high powered ambassadors and generals, whose helicopter wheels let alone feet barely touch the ground.

Afghanistan could still go either way, but the indicators from my snapshot visits round Helmand this past week are not all negative. The problem is that the argument is likely to be won and lost in the dining rooms of London and Washington and not in the fields and bazaars of Afghanistan. This is being conditioned by the enormous gap of perception between the metropolitan commentators at home and the reporters and workers out on the ground here. We are not so much worlds apart, but operating on different planets.